The present invention relates to a coating for optical fibers and a method of manufacturing thereof. More particularly the invention proposes a coating for optical fibers having graded properties so as to improve its resistance under mechanical constraints.
When optical fibers are subjected to mechanical constraints, their optical properties can decrease substantially. One of the main roles of a coating is that of protecting the fibers from such external perturbations. Thus the problem of obtaining a coating structure which could provide such protection in an efficient manner has existed in the related art.
A known solution is a single homogeneous layer for said coating. However as it is known, such single layer coating is not capable of providing an optimum resistance under a relatively high mechanical constraint. Due to theses drawbacks, single layer coating was replaced by a double coating technique using two different UV (ultraviolet) curing resins for coating, so as to provide a higher protection rate. In such structure, an internal layer having a glass transition temperature (Tg) of no more than about xe2x88x9220xc2x0 C. and preferably about xe2x88x9230xc2x0 C. or even lower, and a Young""s modulus lower than 2 MPa, preferably 1 MPa or even lower, prevents microbending and an external layer with a relatively high Tg, generally approximately 50xc2x0 C. and a modulus of about one hundred times higher provides good resistance for fibers handling and cables manufacturing process and installation.
Although the double layer structures have provided improvements in the mechanical properties of the fiber optic coatings, they still present the certain drawback due to the fact that in order to obtain the appropriate mechanical properties the thickness of the overall coating is increased which is not a desirable consequence. Another drawback of such structure is that the associated costs for manufacture of the double layer coating become substantially high.
It is therefore desired to obtain a coating structure such that while the mechanical properties are appropriate so as to resist mechanical constraints, the overall thickness of the coating as well as its manufacturing costs are maintained at relatively low levels.
In order to overcome the above drawbacks, the coating for optical fibers and the method of manufacturing thereof, objects of the present invention are proposed. A typical optical fiber structure has a central core section which is the optical fiber itself, covered by a cladding section for example of silica.
The coating as such covers the cladding section surrounding the outer surface of the latter. The coating proposed by the invention is of a so called graded modulus profile, meaning that the coating is configured so as to present a graded mechanical property as one moves outward from the interface between the cladding section and the coating material toward the outer (external) surface of the coating. The graded properties may thus be obtained by different procedure as will be explained in more detail further below.
Accordingly one objet of the present invention is that of providing a coating for an optical fiber, comprising at least one partly crosslinked polymer, characterized in that the level of crosslinking increases according to the radial distance from an internal surface of the coating which interfaces with a cladding section of the fiber to the external surface of the coating.
According to an aspect of the present inventions there is provided the coating comprises a single resin characterized in that said resin is composed of a volatile, multifunctional and moderately reactive monomer dissolved in a fast curable oligomer, having a low modulus when polymerized.
A further object of the present invention is that of providing a method of manufacturing of a coated optical fiber comprising the steps of drawing of the fiber, deposition of the coating, cure of the coating and spooling of the optical fiber, characterized in that the coating comprises a single resin composed of a volatile, multifunctional and moderately reactive monomer dissolved in a fast curable oligomer, having a low modulus when polymerized and in that said monomer is not completely cured.
According to an aspect of the present invention said volatile monomer is evaporated before without being completely cured.
According to another aspect of the present invention ultraviolet irradiation is used for an incomplete curing of the single resin.
According to yet another aspect of the present invention electron beam irradiation is used for an incomplete curing of the single resin.